Method and System for Controlling Access to Tape Media from a Plurality of Virtual Computers

1. A host computer that accesses a tape library, the tape library comprising: a plurality of tape media; a plurality of slots in which the plurality of tape media are respectively stored; a drive in which a tape medium, among the plurality of the tape media, that has been extracted from any of the plurality of slots is set and which performs reading or writing of data in response to the setting of the tape medium; and a controller by which the tape medium is extracted from a designated slot among the plurality of slots and set in the drive; the host computer comprising: a plurality of virtual computers; a first storage region that stores slot allocation information expressing which slot, among the plurality of the slots, is allocated to which virtual computer; and an access control unit for specifying a slot that is allocated to a first virtual computer among the plurality of virtual computers based on the slot allocation information and for restricting access from the first virtual computer to a specified tape medium, among the plurality of the tape media, stored in the specified slot, wherein the host computer constitutes a cluster with another host computer that accesses the tape library, and, when any of the plurality of virtual computers that are operated by the host computer is down, the virtual computer that is down is restored by the other host computer, the host computer flirt her comprising: a third storage region that stores a host identifier of the other host computer; and a cluster control unit that copies the slot allocation information to the other host computer identified from the host identifier.

2. The host computer according to claim 1 , wherein the tape library comprises a plurality of the drives, a number of the drives being less than a number of the slots, the host computer further comprising: a second storage region that stores drive condition information expressing which drive is in what condition; and a tape library control unit that finds a specific drive in a free condition among the plurality of drives by referencing the drive condition information and transmits to the controller a drive setting instruction for extracting the specified tape medium from the specified slot and setting the specified tape medium in the specific drive.

3. The host computer according to claim 2 , wherein the tape library control unit issues a prescribed query to the controller, determines from a return value in response to the query whether or not the controller can be used, and transmits the drive setting instruction if the controller is identified as being usable.

4. The host computer according to claim 1 , further comprising: a second storage region that stores drive condition information expressing which drive is in what condition; and a tape library control unit that transmits to the controller a drive setting instruction for finding a drive in free condition from the plurality of drives by referencing the drive condition information, and for extracting the specified tape medium from the specified slot and for setting the specified tape medium in the drive found by the tape library control unit, wherein the cluster control unit copies the drive condition information to the other host computer.

5. The host computer according to claim 1 , wherein the host computer is connected to a back-up computer that manages respective back-up conditions of the plurality of virtual computers, and access is executed in response to a back-up instruction from the back-up computer.

6. The host computer according to claim 1 , wherein the plurality of the virtual computers are executed on a single processor.

7. The host computer according to claim 1 , wherein the host computer further comprises an allocation unit which allocates at least one slot, among the plurality of the slots, to each of the plurality of the virtual computers.

8. A method of controlling access to a tape library, the tape library comprising: a plurality of tape media; a plurality of slots in which the plurality of tape media are respectively stored; a drive in which a tape medium, among the plurality of the tape media, that has been extracted from any of the plurality of slots is set and which performs reading or writing of data in response to the setting of the tape medium; and a controller by which the tape medium is extracted from a designated slot among the plurality of slots and set in the drive; the method comprising: a first virtual computer of a plurality of virtual computers running on a single host computer designates an access target, a slot that is allocated to the first virtual computer is specified from slot allocation information expressing which slot, among the plurality of the slots, is allocated to which virtual computer of the plurality of virtual computers, and access from the first virtual computer is restricted to a specified tape medium, among the plurality of the tape media, stored in the specified slot, wherein a cluster is constituted by the host computer and another host computer that accesses the tape library, the slot allocation information is copied from the host computer to the other host computer, when any of the plurality of virtual computers that are operated by the host computer is down, the virtual computer that is down is restored by the other host computer, the slot that is allocated to the virtual computer that has been restored is specified from the copied slot allocation information, and access from the virtual computer that has been restored is restricted to the tape medium in the specified slot.

9. The method of controlling access according to claim 8 , wherein the tape library comprises a plurality of the drives, a number of the drives being less than a number of the slots, a specific drive in a free condition is found among the plurality of drives by referencing drive condition information expressing which drive is in what condition, and a drive setting instruction for extracting the specified tape medium from the specified slot and setting the specified tape medium in the specific drive is transmitted to the controller.

10. The method of controlling access according to claim 9 , wherein a prescribed query is issued to the controller, determination is made from a return value in response to the query whether or not the controller can be used, and the drive setting instruction is transmitted if the controller is identified as being usable.

11. The method of controlling access according to claim 8 , wherein a plurality of the drives are present, drive condition information expressing which drive is in what condition is stored, a drive setting instruction for finding a drive in free condition from the plurality of drives by referencing the drive condition information, for extracting the specified tape medium from the specified slot and for setting the specified tape medium in the drive that has been found, is transmitted to the controller, the drive condition information is copied to the other host computer from the host computer, and a drive setting instruction for finding a drive in free condition from the plurality of drives by referencing the copied drive condition information, extracting the specified tape medium from the specified slot and setting the specified tape medium in the drive that has thus been found, in response to the virtual computer that has thus been restored, is transmitted to the controller.

12. The method of controlling access according to claim 8 , wherein a back-up computer that manages respective back-up conditions of the plurality of virtual computers transmits a back-up instruction to the first virtual computer that requires back-up, among the plurality of virtual computers, and the first virtual computer executes access in response to the back-up instruction received from the back-up computer.

13. The method of controlling access according to claim 12 , wherein a plurality of the drives are present; a drive in free condition is found from the plurality of drives by referencing drive condition information that expresses which drive is in what condition; the back-up computer transmits to the first virtual computer the back-up instruction designating data that requires to be backed up of a plurality of data items managed by the first virtual computer, and the drive that has been found; and the first virtual computer transmits to the controller a drive setting instruction for extracting the specified tape medium from the specified slot and setting the specified tape medium in the drive designated by the back-up instruction, and transmits, to the designated drive, the data designated by the back-up instruction.

14. The method according to claim 8 , further comprising: executing the plurality of the virtual computers on a single processor.

15. A computer system, comprising: a tape library; and a host computer, the tape library, comprising: a plurality of tape media; a plurality of slots in which the plurality of tape media are respectively stored; a drive in which a tape medium, among the plurality of the tape media, that has been extracted from any of the plurality of slots is set and which performs reading or writing of data in response to the setting of the tape medium; and a controller by which the tape medium is extracted from a designated slot among the plurality of slots and set in the drive; the host computer, comprising: a plurality of virtual computers; a first storage region that stores slot allocation information expressing which slot, among the plurality of the slots, is allocated to which virtual computer; and an access control unit for specifying a slot that is allocated to a first virtual computer among the plurality of virtual computers based on the slot allocation information and which restricts access from the first virtual computer to a specified tape medium, among the plurality of the tape media, stored in the specified slot, wherein the host computer is included in a cluster with another host computer that accesses the tape library, and, when any of the plurality of virtual computers that are operated by the host computer is down, the virtual computer that is down is restored by the other host computer in the cluster, the host computer further comprising: a third storage region that stores a host identifier of the other host computer; and a cluster control unit that copies the slot allocation information to the other host computer identified based on the host identifier.

16. The computer system according to claim 15 , wherein the host computer further comprises: a second storage region that stores drive condition information expressing which drive is in what condition; and a tape library control unit that transmits to the controller a drive setting instruction for finding a drive in free condition from the plurality of drives by referencing the drive condition information, and for extracting the specified tape medium from the specified slot and for setting the specified tape medium in the drive found by the tape library control unit, wherein the cluster control unit copies the drive condition information to the other host computer.

17. The computer system according to claim 15 , wherein the plurality of the virtual computers are executed on a single processor.